It is a known practice to adjust the height of the safety belt guide loop so as to ensure that the position of the shoulder-belt portion is adapted to the stature of the user who occupies the corresponding seat.
There is known in particular a device having a guide loop secured to a carriage which is capable of displacement along a substantially vertical rail provided with retaining slots. A locking bolt mounted on the carriage engages in one of the rail slots under the action of a restoring spring.
This adjusting device may present certain hazards if the user has failed to ensure that the carriage is correctly located in a position in which the bolt can engage freely in one of the slots under the action of its restoring spring. Should this not be the case, the carriage is liable to travel very rapidly along the rail in the event of a violent tractive force applied to the safety belt and especially in the event of impact. During this very rapid displacement, the locking bolt is liable to pass over each slot without having time to engage therein, with the result that the travel of the carriage will end only when reaching the limit stop at the lower end of the rail. This corresponds to a considerable increase in length of the two useful portions of the safety belt after locking of the inertia reel. The user is then in serious danger.
This disadvantage becomes still worse in the event that the inertia reel is equipped with a so-called "pretensioning" device. As is already known, a device of this type is started in the event of impact and re-tightens the safety belt. Thus the slack which has developed between the safety belt and the user's body as a result of the limited retraction force of the inertia reel is eliminated before the deceleration caused by an impact becomes fully effective. In consequence, the seat occupant's body is placed in a more favorable position for the remainder of the deceleration. When the pretensioning device is mounted in the vicinity of the inertia reel, it has the effect of applying a downward tractive force on the safety-belt portion which extends from the reel and this tractive force is retransmitted to the safety belt guide loop and consequently to the carriage which supports the guide loop. The force produced by the pretensioning device is therefore directed exactly in the direction in which the carriage slides along the rail in the event that it has been left between two locking positions. In another known arrangement, the pretensioning device is associated with the safety-belt buckle. In this case also, a force having a downwardly directed component is applied by the pretensioning device to the safety belt guide loop via the shoulder-belt portion when the pretensioning device is started. The acceleration of the carriage due to this pretensioning system is thus of considerable magnitude, thereby increasing the danger of its reaching the lower limit stop without any intermediate locking as mentioned earlier.
As disclosed in Patent No. DE-A-31 51019, there is also known a height-adjustment device for a safety belt guide loop in which provision is made for two relatively displaced locking bolts each associated with a respective row of slots, the dimension of each slot in the direction of displacement of the carriage being greater than the corresponding dimension of each locking bolt. The arrangement is such that, in any position of the carriage, at least one of the bolts engages in a slot of the corresponding row. This accordingly ensures that the only uncontrolled displacement of the carriage is a displacement over a maximum distance corresponding to the longitudinal play of the bolt in the slot.
In some preferential positions, the two bolts are engaged so that one bolt is applied against the leading edge of one slot and the other bolt is applied against the trailing edge of the other slot, with the result that there is no longer any play of the carriage in sliding motion along the rail. This device, however, is relatively costly to construct and is subject to a disadvantage in that the action of the two locking bolts is dissymmetrical. In consequence, when one of the bolts is engaged in a slot and is applied on the leading edge of the slot, it produces a tilting torque which is liable to cause a slight pivotal displacement of the carriage and thus to prevent free engagement of the other bolt in the vicinity of the trailing edge of a slot of the other row.
The object of the invention is thus to propose a position-adjusting device for a safety belt guide loop which is more economical to construct and more reliable than the device in accordance with Patent No. DE-A-31 51019.